Excited states theory for optimized orbitals and valence optimized orbitals coupled-cluster doubles models

We introduce an excited state theory for the optimized orbital coupled cluster doubles ~OO-CCD! and valence optimized orbital coupled cluster doubles ~VOO-CCD! models. The equations for transition energies are derived using a similarity transformed Hamiltonian. The effects of orbital relaxation are discussed. We present results for several single-reference molecules (H2O, CH2O, C2H4O, C2H4, BeO!, as well as for molecules with significant nondynamical correlation in the ground state (CH, BH, Ã A1 CH2), and for rectangular O4 1 . We find that: ~i! OO-CCD excitation energies are very close to CCSD excitation energies; ~ii! similarly to the complete active space SCF ~CASSCF! model, the effects of orbital relaxation are very important for VOO-CCD excited states such that the excitation energies calculated by VOO-CCD and CASSCF with orbitals optimized for the ground state are very close to each other and unsatisfactory; ~iii! the VOO-CCD model with an approximate treatment of orbital relaxation describes singly ~valence and Rydberg! and doubly ~valence! excited states within errors of 0.2–1.0 eV at equilibrium geometries and along bond-breaking coordinates; ~iv! the above accuracy of the VOO-CCD model does not degrade as molecules or basis sets grow in size; ~v! the shapes of potential energy surfaces around excited states minima are reproduced well by VOO-CCD model suggesting the use of this method for excited states geometry optimization. © 2000 American Institute of Physics. @S0021-9606~00!30540-2#